1. Field of the Invention
The present invention relates to an optical system for a display apparatus.
2. Description of the Prior Art
There have been so far proposed a number of optical systems for a display apparatus such that read light including image information obtained from a reflection type spatial light modulator is allowed to be incident upon a projection lens to obtain a display image on a screen.
In these display apparatuses as described above, an optical system provided with a reflecting mirror is ordinarily adopted to condense read light emitted from a light source on or near an entrance pupil of the projection lens. However, in the case of a small-sized display apparatus such that a lens of a short focal distance is used as the projection lens and further a large bright image is required to be displayed on a screen positioned relatively near the display apparatus for instance, a lamp of high light intensity is needed for the light source. Consequently, the reflecting mirror for condensing the read light emitted from the light source becomes inevitably large in size and long in focal distance.
Therefore, if a lens of a short focal distance is used as the projection lens to reduce the size of the display apparatus, since a reflecting mirror of a long focal distance is to be used, it is impossible to increase the efficiency of the light source utilization, except when the distance between the projection lens and the reflecting mirror is increased. In other words, the efficiency of the light utilization is reduced in the case of the small-sized display apparatus provided with a reflecting mirror of a long focal distance.
In addition, in the case of such a display apparatus that read light emitted from the light source is allowed to be incident upon the reflection type spatial light modulator through a polarization beam splitter arranged on an optical path between the light source and the projection lens; read light including image information and obtained from the reflection type spatial light modulator is allowed to be incident upon the projection lens through the polarization beam splitter to project the read light on the screen through the projection lens, since the extinction ratio of the polarization beam splitter is dependent upon the incident angle, in the case where the incident light rays traveling from the light source to the polarization beam splitter are not parallel to each other, there arise problems in that the contrast ratio of the displayed image decreases and further the efficiency of the light utilization is deteriorated.
In other words, in the case of the optical system including a reflecting mirror for condensing the read light emitted from a light source on or near the entrance pupil of the projection lens, since the read light allowed to be incident upon the refection type spatial light modulator from the light source through the polarization beam splitter becomes convergent rays as a matter of course, the purity of polarization of the read light allowed to be incident upon the reflection type spatial light modulator inevitably drops. Here, the purity of polarization implies the degree in which a polarized light component having a desired polarization plane is pure and therefore not mixed with the other undesired light components.
Accordingly, when the read light obtained from the reflection type spatial light modulator as the light modulated by image information is allowed to be incident upon the polarized beam splitter, the read light obtained through the polarization beam splitter (i.e., the read light whose intensity is modulated according to image information) is low in contrast. In addition, since the polarized light low in the purity of polarization is generated, the efficiency of the light utilization is low.
To overcome the above-mentioned problems, when parallel light is emitted from the light source and then allowed to be incident upon the polarization beam splitter, it is possible to obtain a display image excellent in contrast ratio. In this case, however, there exists another problem in that a telecentric system is required on the incident side of the projection lens, so that the light intensity of the light source becomes non-uniform on the reflection type spatial light modulator (the shading of the light source appears). Further, when the projection lens having an F-number the same as that of the prior art optical system is required to be used, there exists another problem in that a short focal distance is not obtained and additionally the vignetting of oblique beams occurs, thus deteriorating the efficiency of the light utilization.